1. Field of the Invention
The present invention relates to spindle motors mounted in, for example, information devices or acoustic devices which require precise rotation, relates to bearing units installed in such spindle motors, and relates to production methods therefor, and in particular relates to techniques to be used in HDDs (hard disk drives) in magnetic disk units.
2. Related Art
As precise rotation motors used in, for example, HDDs, there have been provided constructions in which a disk is rotated with high precision by a brushless dynamotor. Bearings suitable for these motors comprise a combination of a radial bearing and a thrust bearing respectively supporting the radial load and thrust load of the rotating shaft. These bearings are generally fluid dynamic bearings in which the rotating shaft is supported with high precision due to the dynamic pressure action of a lubricating oil.
Lubricating oils have generally been generally used as lubricating fluids. Japanese Patent Application, First Publication No. 1995-12127 proposes a bearing using a magnetic fluid. The magnetic fluid is produced by mixing ferromagnetic particles in a lubricating oil, and this magnetic fluid is used in combination with a magnet located in a bearing. In such a structure, the magnetic fluid is usually held in a lubricating pathway by magnetic force, and leakage of the lubricating fluid is thereby restrained. Such a bearing unit is suitable for HDDs in which oil leakage is not desirable. However, the structure in which a magnet is located in a bearing results in an increase of number of parts and a complicated structure, and this is not desirable for the reduction of thickness of motors. Japanese Patent Application, First Publication No. 1999-13765 proposes a structure in which a magnetic powder is mixed and dispersed in a sintered bearing, and then the bearing is magnetized allowing the magnet to be omitted.
In the sintered bearing containing a magnetic powder, relatively hard magnetic powder exists on a bearing surface, and the rotating shaft is not easy to fit thereto and tends to wear easily. Furthermore, the magnetic powder existing on the bearing surface is loosened by wear, and the loosened particles function as an abrasive between the rotating shaft and the bearing. As a result, wear is promoted and vibration characteristics are not stable, precise rotation is not obtained.